Quaternary amine opioid antagonist derivatives have been shown to have utility in a number of contexts. They are considered peripherally acting only, and, therefore, find particular utility in reducing the side-effects of opioids without reducing the analgesic effect of opioids. Such side effects include nausea, emesis, dysphoria, pruritis, urinary retention, bowel hypomotility, constipation, gastric hypomotility, delayed gastric emptying and immune suppression. The utility of these peripherally acting opioid antagonists is not limited to reducing side-effects stemming from opioid analgesic treatment. Instead, these derivatives also have utility in circumstances where endogenous opioids alone (or in conjunction with exogenous opioid treatment) cause undesirable conditions such as ileus and other such conditions including, but not limited to, those mentioned above.
Methylnaltrexone is a quaternary amine opioid antagonist derivative, discovered in the mid-70s. Methylnaltrexone and some of its uses are described in U.S. Pat. Nos. 4,176,186, 4,719,215, 4,861,781, 5,102,887, 5,972,954, and 6,274,591. Stable formulations of methylnaltrexone, however, have heretofore not existed. Methylnaltrexone apparently was assumed to have a structure that was inherently stable. The stability of a pharmaceutical composition in solution, however, is not necessarily predictable either over time when stored at room temperature or when autoclaved.
Naloxone is an opioid antagonist that acts both centrally and peripherally. It differs structurally from methylnaltrexone and would be expected to have a different stability in solution. An allegedly stable formulation of naloxone is described in U.S. Pat. No. 5,866,154.
Surprisingly, it has been discovered that methylnaltrexone is unusually unstable. It further has been discovered that methylnaltrexone has certain degradation products different from those of naloxone. It also has been discovered that critical parameters and conditions are required for stable formulations of methylnaltrexone.